Water heating distribution systems may serve more than one dwelling unit and may have more than one water heating appliance. The energy used by a water heating system is calculated as the sum of the energy used by each individual water heater in the system. Energy used for the whole building is calculated as the sum of the energy used by each of the water heating systems. To delineate different water heating elements several indices are used.
i Used to describe an individual dwelling unit. For instance, CFAi would be the conditioned floor area of the ith dwelling unit. Nunit is the total number of dwelling units.
j Used to refer to the number of water heaters in a system. NWH is the total number of water heaters.
k Used to refer to a water heating system or distribution system. A building can have more than one system and each system can have more than one water heater.
l Used to refer to the lth unfired- or indirectly-fired storage tank in the kth system. NLk is the total number of unfired- or indirectly-fired storage tanks in the kth system. Temperature buffering tanks with electric heating (e.g., minitanks) shall not to be treated as unfired or indirectly-fired storage tanks.
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Symbol |
Definition |
Notes |
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CFA |
Conditioned floor area, ft2 |
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NFloor |
Number of floors in building |
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Nunit |
Number of units in building |
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NK |
Number of water heating systems |
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NWHk |
Number of water heaters in kth system |
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NLoopk |
Number of recirculation loops in kth system (multi-unit dwellings only) |
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CFAi |
Conditioned floor area of ith dwelling unit |
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CFAUk |
Average unit conditioned floor area served by kth system, ft2 |
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